"What a coming-of-age ritual: throw a 16-year-old warrior at some random galactic battle, then give them a pat on the back and their own sword if they survive."

April, 1242: the Doctor and Ruby answer a distress call sent from medieval Russia. The signal's sender? Ranavere, an alien girl forced to take part in a barbaric conflict between the armies of Estonia and Novgorod on the frozen surface of Lake Peipus.

Ranavere wants to escape, but her distress call has summoned her warmongering sisters, intent on preserving family tradition whatever the cost. And as human battle begins, the Doctor and Ruby must face a more devastating threat - a monstrous entity with plans of conquest, growing stronger beneath the icy lake?